Abberlaine Arrol - wearing a long grey skirt and short grey jacket, her hair gathered up inside an official-looking cap - is here to draw the chaotic scene. Her free-hand sketches and water-colours of railway subjects already adorn several boardrooms and office foyers; she is considered a promising artist.

She hands my handkerchief to me. There is something curious in her eyes and stance; I glance at my laundered kerchief and stuff it into a spare pocket. Miss Arrol smiles, to herself, not to me. I have the disquieting impression that I have missed something here.

'Thank you,' I say.

'You may carry my easel for me, Mr Orr; I left it over here last week.' We cross several tracks to a small shed near the centre of the broad, rail-tangled platform of the marshalling yard. Around us, linked carriages and decoupled engines shift slowly back and forth; in other places, whole engines sink slowly through the deck on massively pulleyed platforms transporting them to workshops beneath the tracks.

'What do you make of our strange balloons, Mr Orr?' she asks me as we walk.

'I assume they're there to stop aircraft, though why they're only on one side, I don't know.'

'Nobody else seems to either,' Miss Arrol says thoughtfully. 'Probably another administration cock-up.' She signs. 'Even my father hadn't heard anything about them, and he's usually fairly well informed.'

At the small shed she retrieves her easel; I transport the A-shaped structure to her chosen vantage point. She appears decided upon one of the heavy engine hoists as her choice of subject. She adjusts the easel, sets down her small folding stool, opens her satchel to reveal bottles of paints and a selection of pencils, charcoal sticks and crayons. She surveys the scene with a critical eye, and chooses a black length of charcoal.

'No further ill-effects from our little crash the other day, Mr Orr?' She makes a line on the grey-white paper.

'A certain conditioned nervousness at the sound of rapid rickshaw heel-horns, no more.'

'A temporary symptom, I'm sure.' She unleashes a smile of quite stunning charm, before returning to the easel. 'We were talking about travelling, before we were so rudely interrupted, weren't we?'

'Yes I... I had been going to ask you how far you'd travelled.'

Abberlaine Arrol adds a few small circles and arcs to her sketch.

'University, I suppose,' she says, quickly brushing a few intersecting lines across the paper. 'That was about ...' she shrugs, 'a hundred and fifty ... two hundred sections away. Cityward.

'You ... couldn't see land from there, could you?'

'Land, Mr Orr,' she says, glancing at me, 'my, you are ambitious. No, I could see no land, apart from the usual islands.'

'Do you think there is no Kingdom then, no City either?'

'Oh, I imagine they both exist somewhere.' More lines.

'Haven't you ever wanted to see them?'

'Can't say I have, at least not since I stopped wanting to be a train driver.' She shades areas of the sketch. I can see a succession of vaulting Xs, a suggestion of cloud-shrouded heights. She draws quickly. At the nape of her pale, slender neck some escaped black strands of hair curl over the creamy suface like intricate swirls of some unknown writing.

'You know,' she says, 'I knew an engineer once - quite senior - who thought what we really lived on wasn't a bridge at all, but a single huge rock in the centre of an uncrossable desert.'

'Hmm,' I say, uncertain how to take this. 'Perhaps it is something different to all of us. What do you see?'

'Same as you,' she says, turning briefly to me. 'A bloody great bridge. What do you think I'm drawing here?' She turns back.

I smile. 'Oh, a bit less than a fathom, at a guess.'

I hear her laugh. 'And you, Mr Orr?'

'My own conclusions.' This earns me one of her flashing smiles. She goes back to the drawing for a while, then looks distractedly up for a moment.

'You know the thing I miss most about university?' she says.

'What's that?'

'Being able to see the stars properly,' she nods, looking thoughtful. 'Too much light to see the stars properly round here, unless you go out to sea. But the university was stuck in the middle of farming sections, and it was quite dark at night.'

'Farming sections?'

'You know,' Abberlaine Arrol steps back, arms folded, from her work. 'Places where they grow food.'

'Yes, I see.' It had not occurred to me that other sections of the bridge might be given over to farming; it would not be difficult, I suppose. One might require windbreaks, perhaps even mirrors, to farm on different levels, and water rather than soil would be the best growing medium, but it would be possible.

So the bridge may well be totally self-sufficient in food. My idea that its length was limited by the time it would take for a fast goods train to deliver fresh produce each day would appear to be irrelevant. The bridge can be any length it likes.

Abberlaine Arrol lights a thin cigar. One booted foot taps on the metal deck. She turns to me, folding her arms again beneath the bloused and jacketed contours of her breasts; her skirt swings, comes back; a heavy expensive cloth. There is a hint of a light day-perfume over the fragrant cigar smoke. 'Well, Mr Orr?'

I inspect Miss Arrol's finished drawing.

The broad platform of the marshalling yard has been sketched in, then altered; the lines and tracks look like creepers in a jungle, all fallen to the floor. The trains are grotesque, gnarled things, like giant maggots or decaying tree trunks; above, the girders and tubes become branches and boughs, disappearing into smoke rising from the jungle floor; a giant, infernal forest. One engine has become a monster, rearing out of the ground; a snarling, fiery lizard. The small, terrified figure of a man runs from it, his miniature face just visible, twisted into a shriek of terror.

'Imaginative,' I say, after a moment's thought. She laughs lightly.

'You don't like it.'

'My tastes are too literal, perhaps. The quality of drawing is impressive.'

'I know that,' Miss Arrol says. Her voice is sharp, but her face looks a little sad. I wish I liked the sketch better.

But what a signalling capacity Miss Abberlaine Arrol's grey-green eyes possess! They view me now with an expression almost of compassion. I think that I like the young lady very much.

She says, 'I meant it for you.' She reaches into her satchel and takes out a rag, begins to clean her hands.

'Really?' I am genuinely pleased. 'That's very kind of you. Thank you.'

She takes the drawing from the easel and rolls it up. 'You have my permission to do whatever you will with it,' she says wryly. 'Make paper aircraft out of it.'

'Certainly not,' I say as she hands it to me. I feel as though I have just been presented with a diploma. 'I'll have it framed and hung in my apartment. Already I like it a great deal more, knowing that it was drawn for me.'

Abberlaine Arrol's exits amuse me. This time she is picked up by a track engineer's car; a quaint, elegantly glassed and panelled carriage full of complicated but archaic instruments, all bright brass, clinking balances, and drums of paper flowing past scribbling pens. It hisses and rattles to a stop, a door concertinas open and a young guard salutes Miss Arrol, who is on her way to lunch with her father. I hold her easel, having been instructed to replace it in the shed. Her satchel bulges with rolled line drawings: the commissioned work she really came here to do and which she has been busy with - while still talking to me - since completing my sketch. She puts one boot on the high step up to the carriage and holds out her hand to me.

'Thank you for your help, Mr Orr.'

'Thank you for my drawing.' I take her hand. Between Miss Arrol's boot top and skirt hem her stocking is revealed for the first time; a fine but unmistakable black fishnet.